1. Field of the Invention
The present invention relates to a belt device including a transfer belt for bearing, for example, a toner image and an image forming apparatus provided with the same.
2. Description of the Related Art
An image forming apparatus such as a printer, a facsimile machine or a copier includes, as main constituent elements, photosensitive drums on which toner images are to be formed based on image information from the outside, a belt device including a transfer belt to which a toner image is to be transferred from the photosensitive drum, a transfer unit for transferring a toner image on the transfer belt to a recording medium such as a sheet and a fixing unit for fixing a toner image on a sheet to the sheet.
A belt device generally includes a drive roller connected to a specified drive source, a plurality of driven rollers and a transfer belt mounted on these rollers. The transfer belt has a toner image transferred from the photosensitive drum while being driven and rotated as the drive roller is rotated.
In the belt device, the transfer belt may move in a belt width direction to meander or to be shifted toward one side during the rotation. If the transfer belt meanders or is shifted toward one side, the positions of color toner images are displaced from each other upon transferring a plurality of color toner images one over another to the transfer belt, which causes color drift. As a result, it becomes difficult to form a high-quality image.
In order to solve such an inconvenience, the meandering or shift of the belt needs to be corrected. A first prior art and a second prior art are known as such a technology. A belt device of the first prior art first executes a first control for correcting the meandering based on a positional deviation between the running position and targeted running position of a belt to make the positional deviation fall within a specified range and then executes a second control for correcting the meandering of the belt based on a deviation between an average value, calculated by sampling the running position of the belt a plurality of times, and the targeted running position, thereby suppressing color drift.
A belt device of the second prior art changes a control parameter, e.g. changes the detection interval of the belt running position from a short interval to a long interval, to correct the meandering of the belt for suppression of color drift when at least two of a belt meandering amount, a meandering change amount and a meandering speed fall to or below correspondingly set specified values.
However, in order to suppress the color drift, it is necessary to switch the control from the first control to the second control in the belt device of the first prior art and to change the control parameter in the belt device of the second prior art, wherefore the color drift suppressing control is complicated.